Pseudobinary mercury, cadmium, telluride (HCT) alloy crystals are uniquely suited for use in infrared detectors for a variety of reasons. HCT crystals have the unique property that the energy gap can be varied from 0 to 1.5 eV by varying the composition. The spectral response of an infrared detector incorporating the crystals can be matched to any wave length in the range of 1 to 50 .mu.m at working temperatures at or above 77.degree. K. The HCT alloys produce semiconductors having high absorption coefficients with low intrinsic carrier concentrations and high electron mobilities. These valuable properties of HCT alloys have been recognized for many years, but at present, the potential of such alloys and of detectors and other devices incorporating such alloys has been limited by a variety of difficulties in growing crystals of sufficient homogeneity and reasonable size.
The standard method of producing HCT crystals is by solid state recrystalization. In solid state recrystalization, a solid of uniform composition is formed by melting the alloy constituents together and quenching. However, quenching introduces stress and crystal defects and causes microscopic lack of homogeneity which makes the crystals useless as good detectors, as formed. In order to utilize crystals formed by the solid state recrystalization method, it is necessary to anneal the crystals for long periods of time to obtain a composition which has an equilibrated structure and a satisfactory uniformity of composition. Six to eight weeks is usually required in the annealing process. Even then, the overall yield of crystals obtained by this method is quite low.
Recently, some interest has developed in using a modified Bridgman-Stockbarger method of crystal growth in order to obtain crystals of uniform composition and having a low incidence of sub-microscopic crystal defects, or localized inhomogeneity. A recent discussion of this method can be found in the Journal of the Electrochemical Society, Volume 125 (1978) beginning at page 315, entitled "A Possible Method for the Growth of Homogeneous Mercury Cadmium Telluride Single Crystals." It has been found, however, that the Bridgman-Stockbarger method tends to create crystals which do not have a satisfactory homogeneity, both in the axial direction along the major axis of the ampule in which the crystal is formed and also radially outward from the axis of the crystal. As a result, crystals formed by this method have not been acceptable as crystals for infrared detectors, for example, due to the extremely low yield of suitable crystals formed by this method. Moreover, the Bridgman-Stockbarger method has not been suitably reliable; it has not given reproducible results when used to grow HCT crystals.
Applicants have discovered a method of controlling the crystalization parameters of a modified Bridgman-Stockbarger method which enables homogeneous crystals to be reproducibly formed by that method, both in a radial and axial direction. Applicants' method may be utilized to produce HCT crystals for devices requiring a uniform spectral response, for example, in infrared detectors, but may also be used to produce other alloy crystals as well.